A portable, battery-operated digitizer tablet computer has been designed which includes a main or host processor that executes one or more application programs under a pen oriented operating system. The computer includes a digitizer coupled with a display that is intended to be the primary output device for interacting with a user. A pen or stylus produces a magnetic field which is sensed by a digitizer to provide signals representing the position of the stylus. The stylus is used as the primary input device for entering information, including handwriting, into the computer. The analysis of handwriting requires a high precision digitizer which in turn generates a multiplicity of coordinate data and thereby creates a heavy signal processing load. To meet this load, the computer has distributed processing in which the initial handling of digitizer information is off loaded onto a service processor from the main processor. The service processor is also used to control a power subsystem and a keyboard that can optionally be connected to the computer. The present invention is directed to the interface between the host processor and the service processor which interact to control operation of the various computer components while permitting digitization to be done rapidly and in an efficient manner.